Methods of making protective coating for metal surfaces

ABSTRACT

A coating for a metal surface that provides excellent resistance to both electrochemical corrosion and mechanical insult is provided. The coating involves at least an inner coating that is a sacrificial anodic layer and an outer coating that is a protective dielectric material made of inorganic metal oxide. Some versions of the coating include an intermediate layer as well that serves to improve adhesion between the coatings and may provide additional galvanic protection. Although the coating can be made by a variety of methods, advanced methods of spray application are provided for making high-quality lightweight versions the coating.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application under 35 U.S.C. §120 ofU.S. application Ser. No. 12/690,568, filed Jan. 20, 2010 (currentlypending). U.S. application Ser. No. 12/690,568 is hereby incorporated byreference in its entirety.

BACKGROUND

A. Field of the Disclosure

The present disclosure relates generally to advanced protective coatingsfor metal surfaces. Such coatings as well as methods of making and usingthem are provided in this disclosure.

B. Background

Corrosion is an electrochemical process that occurs when an electricallyconductive metal structure is in contact with an electrically conductivesubstance, such as water. In such situations a current leaves astructure at an anode site, passes through an electrolyte, and reentersthe structure at a cathode site. The electric current causes chemicalchanges in the structure, converting the metal to a structurally unsoundcorrosion product. As an example, one small section of a pipeline may beanodic because it is embedded in soil that is more conductive than thesoil along the rest of the pipeline. Current flows between the anode andcathode facilitated by the conductive pipe and the electrolytic soil. Inthe process, the metal in the pipe undergoes a shift in oxidation statethat results in electrochemical corrosion.

Cathodic protection is a method to reduce corrosion by minimizing thedifference in potential between anode and cathode. This is achieved byapplying a current to the structure to be protected from some outsidesource. When enough current is applied, the whole structure will be atone potential; thus, anode and cathode sites will not exist. Cathodicprotection is commonly used on many types of structures in corrosiveelectrolytic environments, such as pipelines, underground storage tanks,locks, and ship hulls.

There are two main types of cathodic protection systems: galvanic andimpressed current. A galvanic cathodic protection system makes use ofthe corrosive potentials for different metals. Without cathodicprotection, when one area of the structure has a higher negativepotential than another, corrosion results. If, however, a much lessinert object (that is, an object with much more negative potential) isplaced adjacent to the structure to be protected, such as a pipeline,and an electrical connection exists between the object and thestructure, the object will become the anode and the entire structurewill become the cathode. The new object corrodes instead of thestructure thereby protecting the structure. In this example, the objectis called a “sacrificial anode.” Thus, the galvanic cathodic protectionsystem is also called a “sacrificial cathodic protection system” becausethe anode corrodes “sacrificially” to protect the structure. Galvanicanodes are made of metals with higher negative potential than the metalof the structure itself; the metal of the anode is said to be “anodic”compared to the metal of the structure.

Attachment of the anode is normally done at the jobsite utilizingunderground insulated wire, thermite weld, conventional weld, orthreaded bolts. Problems related to these methods of attachment to thestructure include, but are not limited to: improper placement of theanode, improper size of the anode, improper composition of the anode,damage to the metallic object or the internal lining from excessive heatfrom the weld, loss of structural integrity of the metallic object,damage to the anode, wire, or electrical connection during installationand backfilling operations, improper weld or connection at the jobsiteresulting in the loss or reduction in effectiveness of protection, andfailure to remove protective wrapping from the anode prior to burial.Traditional sacrificial anode placement of buried structures alsorequires extra trench excavation either several feet below the structureor several feet to the side of the structure. As a result, there is along-felt need in the art for an effective means of galvanic protectionwithout these limitations.

One method of protection which solves the problems listed above is ametallic sacrificial coating applied directly to the structure'ssurface. The coating acts as a barrier between the metal in thestructure and the environment, and if breached it acts as a sacrificialanode to prevent corrosion. However, traditional methods of providing asacrificial coating have several drawbacks. Traditional methods, such ashot-dip galvanizing, electroplating, thermal diffusion galvanizing, andvapor galvanizing are expensive, energy-intensive, and time-intensive;in addition, most traditional methods provide poor control of thethickness and consistency of the anodic coating. Accordingly, there is along-felt need in the art for methods of applying corrosion protectionthat allow rapid, inexpensive, thin yet complete coatings to be applied.

As materials become increasingly expensive, there is a need to providethin yet effective galvanizing layers. Although thin layers of anodicmetal have the advantage of lower weight and cost, they have thedisadvantages of being easily damaged or worn off. An anodic layer canbe protected by a barrier layer, but over time barrier layers have atendency to delaminate and allow corrosive material to contact theanodic layer. Accordingly, there is a long-felt but unmet need in theart for a galvanically protective coating paired with a barrier coatingthat will be durable, long-lasting, and with superior adhesioncharacteristics.

SUMMARY

The disclosure provides novel and useful protective coatings for metalsurfaces, methods of making them, methods of using them, and apparatusescomprising them. The following goals are met by at least someembodiments of the processes, machines, manufactures, compositions ofmatter, and other teachings of the instant disclosure: to providegalvanic protection to a metal surface that does not requirepost-manufacturing assembly or construction; to provide galvanicprotection to a metal surface rapidly and economically; and to provide agalvanic coating and a dielectric coating that will have superioradhesive qualities.

These and other goals are met by providing a coated metal surfacecomprising: a metallic substrate; an outer coating comprising an oxideof a second metal, wherein the oxide of the second metal is dielectric;and an inner coating disposed between the substrate and the outercoating, the inner coating comprising a first metal that is anodic tothe substrate, wherein the first metal is substantially electricallyconnected to the metallic substrate. These and other goals are also metby providing a process (and the product thereof) of coating a metalsurface, comprising: applying an inner coating to a metal substrate, theinner coating comprising a first metal that is anodic to the substrate;and applying an outer coating to the side of the inner coating oppositeto the substrate, the outer coating comprising an oxide of a secondmetal, wherein the oxide of the second metal is dielectric; and whereinthe first metal is substantially electrically connected to the metalsubstrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: This figure depicts a coated underground pipe and valve.

FIG. 2: This figure shows a cutaway view of the pipe of FIG. 1 toillustrate the components of the coating.

FIG. 3: The figure shows a cutaway view of an embodiment of the coatedmetallic surface in which the intermediate coating is a compositecoating.

DETAILED DESCRIPTION A. Definitions

The terms “include” and “including” as used herein are non-limiting, andcan be read to mean “including but not limited to.”

The term “anodic” as used herein refers to the property of having a morenegative electrical potential than some reference substance. Unlessotherwise stated, in this disclosure the reference substance is ametallic substrate that is the subject of galvanic protection.

All terms in the singular should be read to include the plural, andvice-versa, unless otherwise stated. Similarly, any male pronouns shouldbe read to include the female, and vice-versa, unless otherwise stated.Unless stated otherwise, terms should be read to have their accepteddictionary definitions.

B. Coated Metal Surfaces

As explained above, metal surfaces exposed to an electrolyte are subjectto electrochemical corrosion, particularly if the surface developsdifferentially charged regions such that some parts of the surfacefunction as cathodes and others as anodes. Corrosion can be deterredusing a galvanic cathodic protection system. Galvanic coatings havenumerous advantages over other forms of anode. However, galvaniccoatings, if damaged, can expose the underlying metal substrate.Although the underlying metallic substrate will still be galvanicallyprotected, it will be subject to more rapid corrosion than otherwise andis vulnerable to further mechanical damage. For this reason, it isdesirable to provide a coating to protect the surface from mechanicalinsult, to prevent contact with corrosive substances in the environment,and to insulate the substrate from stray electrical currents. Byseparating the metallic substrate from corrosion-inducing aspects of theenvironment, non-galvanic “barrier” coatings can also be effective inpreventing corrosion.

These and other goals can be achieved by providing a metallic substrateto be protected, an outer coating comprising a dielectric metal oxide,and an inner anodic coating between the substrate and the outer coatingthat is electrically connected to the substrate. The inner coatingcomprises a first metal that is anodic with respect to the substrate.The outer coating comprises a dielectric oxide of a second metal,wherein the second metal may be the first metal or a metal other thanthe first metal. The inner coating is substantially electricallyconnected to the metal substrate, such that it provides galvanicprotection to the substrate. The dielectric metal oxide provides atleast partial protection from electrical currents, the corrosiveenvironment, and mechanical damage. In some cases it is desirable toprovide an intermediate coating comprising a third anodic metal.

The coated surface, and any individual coating, may also be the productof any of the processes described below.

1. Metallic Substrate

The metallic substrate can be any metal that is subject toelectrochemical corrosion, as known by those skilled in the art. Becauseunderground and submerged structures are particularly subject tocorrosion, metals that are commonly used in underground and submergedstructures are advantageously protected by the coating. In pipes themetal is typically ferrous metal, steel, iron, iron alloys, lead,nickel, brass or copper. However, the utility of the coatings is notlimited to these metals or to any particular applications. The metallicsubstrate will often be a portion or a surface of a greater system orstructure, as is described more completely below.

2. Inner Coating

An inner coating is present between the outer coating and the substrate.The inner coating may cover the entire substrate, or only a portion ofthe substrate. The portion will often be the part of the substrate thatis most likely to encounter corrosive conditions (such as the exteriorsurface of an underground pipe or the interior surface of a conduit fora strong electrolyte). The thickness of the inner coating will depend onseveral factors. Thinner inner coatings have the advantages of lowercost and lower weight. Thicker inner coatings have the advantage ofgreater durability and longevity. If the coating as a whole is toothick, assembly of coated parts becomes more complicated or impossible.For example, it has been found that coatings on fluid conduit componentparts should ideally not exceed about 10 mils (25.4 μm) in totalthickness for ease of assembly and seal integrity. A good balancebetween the competing advantages of a thicker inner coating versus athinner inner coating may be for example 2-3 mils (50.8-76.2 μm), orabout this range. The inner coating will typically be porous or somewhatporous depending on the method of application, although the innercoating can still provide galvanic protection if it is non-porous.

The inner coating is composed of a first metal that is anodic incomparison to the substrate (a “first anodic metal”). This can be anymetal that is anodic to the metal of the substrate. For ferroussubstrates, the first anodic metal may comprise for example zinc,aluminum, magnesium, indium, gallium, tellurium, or alloys of one ormore of the foregoing. Some types of first anodic metal have theadvantage of forming oxides upon sacrificial oxidation that function toprotect the underlying metal from further oxidation; most advantageouslysuch metals will form oxides that are insoluble, tenacious (adherefirmly to the metal), and do not readily form hydrates. Such propertieswill be influenced by the conditions surrounding the coating, as isunderstood by those skilled in the art. The inner coating will oftencomprise a very high content of the anodic metal, for example 50%, 90%,95% 99%, 99.9%, or about these values. The alloys may compriseadditional elements, for example carbon or silicon, to enhance theadhesion of the coating to the substrate or other properties, as isunderstood by those skilled in the art. Particularly useful anodicmetals for ferrous substrates are substantially pure zinc, substantiallypure magnesium, substantially pure aluminum, aluminum-zinc alloy,aluminum-silicon alloy, and indium alloy. Indium alloys are particularlyuseful in high salinity environments. An aluminum-zinc alloy can be usedthat comprises, for example, 85 parts zinc to 15 parts aluminum (orabout this ratio). An aluminum-silicon alloy can be used that comprises,for example, 88 parts aluminum to 12 parts silicon (or about thisratio). In some embodiments of the coating, the inner coating mainlycomprises metallic zinc, as zinc is inexpensive, adheres well to ferroussubstrates, and provides excellent anodic protection to ferroussubstrates. Some embodiments of the inner coating comprise at leastabout 50%, 90%, 95% 99%, 99.9%, or 100% metallic zinc (or at leastexactly these values). Some embodiments of the inner coating essentiallyconsist of metallic zinc. Some embodiments of the inner coating mainlycomprise metallic aluminum, as aluminum is inexpensive, durable, forms atenacious protective oxide layer upon oxidation, and provides goodanodic protection to ferrous substrates. Some embodiments of the innercoating comprise at least about 50%, 90%, 95%, 99%, 99.9%, or 100%metallic aluminum (or at least exactly these values).

The inner coating can be applied by any method known in the art, or byspraying techniques described below. The spraying techniques have theadvantage of applying a thin consistent layer rapidly at low cost.Adhesion can in some cases be improved by application of an underlayerto the metallic substrate prior to application of the inner coating. Theunderlayer is preferably very thin, such as a flash coating. Forexample, the adhesion of an aluminum inner coating to a ferroussubstrate can be greatly improved by the application of a zincunderlayer flash coating. Zinc provides excellent adhesion, createslittle pollution, and is relatively inexpensive. An underlayer flashcoating of cadmium also provides excellent adhesion characteristics.Other flash coatings that enhance the adhesion of certain anodic metalsto certain substrate metals are known in the art, and can be used in thecontext of the coated metal surface.

It is critical that the inner coating is at least partially electricallyconnected to the substrate. In some embodiments of the coating, theinner coating is substantially electrically connected to the substrateor completely electrically connected to the substrate. In this context,“substantially electrically connected” means that a substantial portionof the first metal can convey a current to the substrate. The innercoating may be at least partially in contact with the substrate. Suchcontact is one means of providing the electrical connection between theinner coating and substrate. In some embodiments of the coating, theinner coating is in complete contact with the substrate (contacting thesubstrate at substantially every area where corrosion protection may benecessary).

3. Outer Coating

The outer coating functions as a barrier, preventing contact from thelower coatings and substrate to the potentially corrosive environment.This includes preventing contact with electrolytes and other aggressivefluids. The outer coating also functions as a dielectric coating,preventing contact with unwanted electrical currents. The outer coatingmay also function to prevent contact with solid objects that wouldotherwise wear away the other coatings and substrate, or conductunwanted electric currents to the other coatings and substrate. Theouter coating can also function to protect the other coatings and thesubstrate from wear resulting from contact with flowing fluids, with orwithout suspended abrasive particles. It is advantageous if the outerlayer is at least one of: dielectric, resistant to wear, resistant toimpact, resistant to fracture, and flexible. The outer layer may bepermeable to corrosion products, electrolytes, or both. It is alsoadvantageous if the outer layer is inexpensive and amenable to advancedcoating techniques, such as thermal spraying and cold spraying.

Metal oxides have many advantages over other types of dielectricmaterials, such as commonly used organic polymers. Metal oxides can beextremely hard, and in fact are among the hardest materials known. Assuch they are generally more resistant to abrasion and impact than arepolymers. Metal oxides do not oxidize even under highly oxidizingconditions, whereas many commonly used polymers are subject to oxidationand degradation, for example during storage or weathering. Metal oxideshave extremely high temperatures of fusion, and can be applied to aworkpiece at a much higher workpiece temperature than can organicpolymers; furthermore, metal oxides will not burn if applied at hightemperature in an oxidizing atmosphere. This allows the outer coating tobe applied to a metallic workpiece shortly after annealing, withoutdelay to allow the workpiece to cool. Many metal oxides are insoluble inboth hydrophobic and hydrophilic solvents (particularly at lowtemperatures), unlike most organic polymers which may dissolve ordegrade in contact with hydrophobic solvent. This allows metal oxides tobe used for applications such as coatings for the interiors of storagetanks for organic solvents or conduits for organic solvents.Furthermore, the synthesis and application of metal oxides does notinvolve emissions of volatile organic compounds, nor do they containresidual VOC after application; as a result, they are less polluting andpose less hazard in the workplace than do organic polymers.

The outer coating comprises a mechanically resistant dielectricmaterial. In some embodiments of the outer coating, the materialcomprises a dielectric oxide of a second metal. Such dielectric oxidesare known to those skilled in the art. Such oxides include oxides ofsilicon, aluminum, magnesium, and titanium. The outer layer may bemostly comprised of the oxide, and exemplary concentrations of the oxideinclude at least 50%, 90%, 95%, 99%, 99.5%, 99.9%, 100%, and about thesevalues. In some embodiments, the second metal is the same as the firstmetal. Aluminum oxide (Al₂O₃) is one example of a suitable dielectricoxide. It is durable, highly dielectric, abundant, inexpensive, andforms a tenacious passive oxide layer. It is also insoluble in water andhas high resistance against weathering and strong acid. It has goodthermal conductivity compared to most dielectric materials and canreduce thermal shock resistance. The composition of aluminum oxide canbe easily changed to enhance certain desirable material characteristicssuch as the hardness or color. Aluminum oxide is an electricallyinsulating material with high resistivity that increases with purity.With its high corrosion resistance, it does not wear easily, even whenchallenged with strong acid. Aluminum oxide can also be applied tosurfaces by thermal spraying in the form of a powder.

The thickness of the outer layer will vary depending on the structure tobe coated, the environment, the dielectric material used, and otherfactors. A thin layer has the advantage of the efficient diffusion ofcorrosion products from the other coatings and substrate, greaterflexibility, lower weight, lower volumes, and lower cost. A thin layerwill also pose less interference during assembly and installation inusing coated structures due to increased thickness. A thick layer hasthe advantage of more robust and lasting protection. An exemplary outercoating may be 2-6 mils (50.8-152.4 μm) in thickness, or about thisrange.

The outer coating is disposed on the opposite side of the inner coatingfrom the substrate. Some embodiments of the outer coating are in directcontact with the inner coating. Some embodiments of the outer coatingare in direct contact with the intermediate coating described below. Insome embodiments of the coating, there are intervening layers orinterstices between the outer coating and either of the inner coating orthe intermediate coating.

Some embodiments of the outer coating are permeable to the corrosionproducts of the coating underlying the outer coating (the inner orintermediate coating). The corrosion of the underlying coating can insome cases cause blistering and spalling of the outer coating. Thisoccurs when the surface of the underlying coating that is in contactwith the outer coating corrodes to form a corrosion product that haspoor mechanical properties, swells due to the formation of a hydrate, orthat is soluble in the medium. The corrosion product may also occupy agreater molar volume than the non-corroded material of the underlyingcoating, causing blistering (this is generally true in the case ofhydrates). As there is no longer a solid surface of the underlyingcoating to which the outer coating can adhere, the outer coating willspall from the underlying coating. This blistering and spalling can beprevented or reduced by providing an outer coating that is permeable tothe corrosion product. Corrosion products will diffuse in some casesfrom the interface between the outer coating and the underlying coatinginto the medium, preventing blisters. Permeability may be due to thecomposition of the outer coating, the thickness of the outer coating, orthe porosity of the outer coating. Metals that form soluble oxides havethe advantage of being capable of aqueous diffusion if the outer coatingis permeable. Permeability may be increased by providing a thin outercoating or a porous outer coating.

Alternatively, an impermeable outer coating can be used. An impermeableouter coating has the advantage of preventing contact of the lowercoatings with the environment. Such impermeable outer coatings willtypically be non-porous and sufficiently thick to prevent diffusion.

Some embodiments of the outer coating are permeable to the medium incontact with the outer coating. The medium will generally be air, soil,or water. However, the medium can be nearly any form of matter dependingon the application. Other examples of media include compressed naturalgas, hydrocarbon fuel, liquefied gasses, refrigerant, biological fluids,or any other fluid material. Under some conditions, by allowing theanodic metal in one or more of the underlying coatings to contact themedium anodic protection can be provided to the substrate. Again,permeability may be due to the composition of the outer coating, thethickness of the outer coating, or the porosity of the outer coating.

Some embodiments of the outer coating comprise an additive that providesimproved properties. Such properties include, for example, suitabilityfor spray application, resistance to abrasion, hardness, strength,toughness, elasticity, plasticity, brittleness, ductility andmalleability. Examples of additives include a fluxing agent (whichremoves impurities), an agent to lower the temperature of fusion, and anagent that improves the flow characteristics of the coating materialduring spraying. Feldspar, for example, improves the application ofmagnesium oxide to surfaces. Silica generally improves the spraycharacteristics of solids by improving flow.

Some embodiments of the outer coating comprise a topcoat. The topcoatserves as a further barrier between the environment and the coatedsurface. The topcoat materials may be organic, inorganic, or acombination of organic and inorganic material. The topcoat willadvantageously be dielectric, impermeable to water (or other medium towhich the coated metal surface is likely to be exposed), or both.Suitable organic topcoats include asphalt, wax, and paint. The topcoatmay serve any of a variety of other purposes, such as: sealinginterstices between grains of oxide, reducing the external roughness ofthe coated metal surface, reducing friction during assembly (for exampleduring joint assembly and horizontal directional drilling), andimproving the appearance of the final product.

4. Intermediate Coating

Some embodiments of the coated metal surface comprise an intermediatecoating. The intermediate coating is disposed between the inner andouter coating, and comprises a third metal that is anodic to themetallic substrate. The third metal is substantially electricallyconnected to at least one of the metallic substrate and the innercoating. This allows the third metal to function as a sacrificial anodeto at least one of the metallic substrate and the inner coating.

The third metal may be any metal that is anodic to the substrate. Thismay include any metal that is disclosed herein as suitable as the firstmetal. The third metal may additionally be anodic to the first metal. Ifthe third metal is anodic to the first metal, the intermediate coatinghas the advantage of providing anodic protection to the inner coating.In some embodiments of the intermediate coating, the third metal is: analloy comprising at least one of the first metal and the second metal,an alloy of the first and second metals, the second metal, or the firstmetal. A third metal that is an alloy of the first and second metals hasthe advantage of providing superior adhesion to both the outer coatingand the inner coating. Exemplary embodiments of the third metal comprisean aluminum alloy, a zinc alloy, at least 50% of one of zinc oraluminum, an alloy of zinc and aluminum, and essentially pure aluminummetal. Aluminum, for example, has the advantage of low cost, durability,good corrosion resistance, and is anodic to most other metals (theexceptions being zinc, beryllium, and magnesium). Both aluminum and zinchave the advantage of forming protective oxides upon corrosion, whichcan create a “backup” oxide layer in addition to the oxide in the outercoating.

The intermediate coating may be a composite coating. Such a compositecoating would be present in at least partial contact with the innercoating. In some embodiments of the composite coating, the third metaland a fourth metal are interspersed in the coating so as to form amatrix of the two substances. Some embodiments of the composite coatingcomprise the third metal and a metal oxide. The metal oxide may be anymetal oxide that is suitable of the metal oxide in the outer coating,including the oxide of the second metal.

The matrix can be of regular configuration or it can be of randomizedconfiguration. The matrix will allow the third metal to contact theinner coating in at least substantial portions of the intermediatecoating. This contact allows the third metal to be electricallyconnected to the substrate through its electrical connection to theinner coating. In other words, when viewed from the side cross-section,it is desirable that the composite intermediate coating be such thateither: (1) the third metal particles are large and the thickness of theintermediate composite coating is small such that the third metalparticles protrude through and are exposed on both sides of theintermediate composite coating or (2) the concentration of the metalparticles is sufficient to result in an electrically continuousconductive path from one side of the intermediate composite coating tothe other. The ratio of components of the composite (such as the thirdmetal to fourth metal, or the third metal to a metal oxide) should beformulated with this in mind.

The third metal can be any metal that is anodic with regard to at leastone of the substrate and the inner coating. The third metal can be anyof the metals described as suitable for the first metal. In some casesthe first metal and the second anodic metal 6 will be the same. This hasthe advantages of simplicity in manufacturing, good galvanic protection,and superior adhesion between the intermediate composite coating and theinner coating.

The fourth metal provides superior adhesion to the coated surface,preventing non-adhesion events such as spalling and blistering. In manycases the superior adhesion will be achieved due to better bondingcharacteristics between the third metal and the oxide of the secondmetal in the outer coating. If the fourth metal is the same as thesecond metal, the fourth metal may also function to regenerate the oxideof the second metal in the outer coating. Should the outer coating bebreached, the fourth metal will react with ambient oxidants to form alayer of metal oxide. The fourth metal may be any metal that isdisclosed to be suitable as the second metal. Two examples, titanium andaluminum, have the advantage of forming oxides that are tenaciouslyassociated with the underlying non-oxidized metal. As a result, astitanium and aluminum weather under oxidizing conditions, a fine oxidelayer forms tightly bound to the underlying metal which protects theunderlying metal from further weathering.

Similarly, the metal oxide in the composite provides superior adhesionwith the metal oxide in the outer coating. The use of the same metaloxide in the composite as is present in the outer coating providesparticularly good adhesion between the intermediate coating and theouter coating.

One simple embodiment of the intermediate layer comprises the firstmetal, and the outer coating comprises an oxide of the first metal.Another simple embodiment of the intermediate layer comprises acomposite of the first metal and an oxide of the first metal, and theouter coating comprises the oxide of the first metal. An example of onesuch embodiment is a coated metal surface comprising an inner layer ofaluminum, an intermediate layer comprising a composite of aluminum andAl₂O₃, and an outer layer comprising Al₂O₃.

As discussed elsewhere in this disclosure, those skilled in the artunderstand that a balance must be achieved between providing a thickcoating with enhanced durability and providing a thin coating with lowerweight and volume. The same balance must be achieved in the intermediatecoating as in the other coatings. An intermediate coating of about orexactly 2-3 mils (50.8-76.2 μm) in thickness is suitable for the coatedmetal surfaces of the instant disclosure.

The intermediate coating may be at least partially in contact with theouter coating or the inner coating (or both). Such contact is one meansof providing the electrical connection between the intermediate coating,the inner coating, and the substrate. In some embodiments of thecoating, the intermediate coating is in complete contact with the innercoating (contacting the inner coating at substantially every area wherecorrosion protection may be necessary). Some embodiments of theintermediate coating are in complete contact with the outer coating.

Some embodiments of the intermediate coating are permeable to thecorrosion products of the inner or intermediate coating. Such corrosioncan in some cases cause blistering and spalling, as described above.Permeability may be due to the composition of the intermediate coating,the thickness of the intermediate coating, or the porosity of theintermediate coating. Metals that form soluble oxides have the advantageof being capable of aqueous diffusion if the intermediate coating ispermeable. Permeability may be increased by providing a thin coating ora porous coating.

Alternatively, an impermeable intermediate coating can be used. Animpermeable intermediate coating has the advantage of preventing contactof the inner coating with the environment. Such impermeable intermediatecoatings will typically be non-porous and sufficiently thick to preventdiffusion.

B. Metallic Structures

The disclosure provides metallic structures comprising any of the coatedmetallic surfaces disclosed herein. This can be a structure that is inperiodic, frequent, or constant contact with an electrolyte. Suchstructures may be underground, submerged, or in extremely humidenvironments. Metallic structures that are submerged include marine andaquatic structures, such as ships, chains, seawalls, flood controlstructures, dams, bridges, fixed and mobile offshore platforms,submarine and submersibles, pipelines, cables, navigation structures,locks, subsea systems, and coastal structures such as piers. Fluidconduits are examples of structures than are frequently in contact withwater, including groundwater, seawater, and open freshwater. Fluidconduits include a pipe, a pipe fitting, a pipe valve, a pump component,a pipe fixture, and an appurtenance to a pipe. One example is pipingthat is subject to corrosion, such as underground piping. Undergroundpiping systems are well known in the art, and include importantinfrastructure systems such as freshwater pipes, sewer pipes, stormdrains, steam pipes, and fuel gas pipes. Such conduits are virtuallyalways located underground between source points (such as treatment orproduction plants) and delivery points. Aboveground pipes may also besuitable substrates, particularly in areas of high humidity orprecipitation.

Other structures that benefit from protection against corrosion are wellknown in the art, any of which could benefit from the coated metalsurfaces disclosed herein.

C. Process for Coating a Metal Surface

Novel processes of coating a metal surface have been developed. Theseprocesses are useful to produce any of the coated metal surfaces of thisdisclosure, as well as other coatings.

The process involves applying an inner coating to a metallic substrate,the inner coating comprising a first metal that is anodic to thesubstrate; and applying an outer coating on the side of the innercoating opposite the substrate, the outer coating comprising an oxide ofa second metal, wherein the oxide of the second metal is dielectric; andwherein the first metal is substantially electrically connected to themetal substrate. Some embodiments of the process include applying anintermediate coating to the inner coating, the intermediate coatingcomprising a third metal that is anodic to the metallic substrate,wherein the third metal is substantially electrically connected to thefirst metal, and wherein the outer coating is applied to theintermediate coating. In some embodiments of the process, at least oneof applying the inner coating, applying the intermediate coating, andapplying the outer coating comprises spray application, for examplethermal spraying.

To facilitate coating of the substrate, the substrate may be abraded orheated. If the substrate is heated, the temperature will typically be upto the fusion temperature of the coating. For certain purposes it isoptimal to heat the substrate to the coating's fusion temperature orabout this temperature. While it is advantageous to heat or abrade thesubstrate for this purpose, it is not necessary in every case.

The coatings may be applied by any methods known in the art. However,there are distinct and unique advantages to the use of advanced spraymethods to apply at least one of the coatings. Advanced spray methodsare inexpensive compared to other coating methods. They require littleinfrastructure, require little time, and can be performed on the site atwhich the metallic substrate is manufactured. Spray techniques canachieve thin yet uniform coatings which are of unique value inrigorously protecting surfaces without unnecessary weight or volume.

Thermal spraying is particularly useful in the disclosed coated metalsurfaces and processes. Any thermal spraying method can be used,including detonation spraying, flame spraying (powder or wire),high-velocity liquid fuel spraying, high-velocity air fuel spraying,high-velocity oxygen fuel spraying, plasma spraying, and arc spraying.Cold spraying and warm spraying techniques can be used; in some casesthe material to be sprayed will be heated prior to application by coldspraying. If the material is applied by thermal spraying, the sprayedmaterial will be in a liquid state at the time it is sprayed. If thesprayed material is applied by cold spraying or warm spraying, thesprayed material may be solid or liquid at the time it is sprayed.

The coated metal surface, substrate, inner coating, intermediatecoating, and outer coating may be any of the substrates or coatingsdescribed herein.

1. Application of Metal Coatings

The inner coating or intermediate coating (“metal coatings”) may beapplied to the substrate or inner coating (“target surface”) by anymethod known in the art. Such methods include hot-dip galvanizing,electroplating, thermal diffusion galvanizing, and vapor galvanizing.The metal coatings may also be applied by advanced spraying techniques.Thermal spraying methods such as flame spraying (wire or powder), arcspraying, and plasma spraying are examples of advanced spraying methodsthat produce good results in applying the metal coatings. Thermalspraying has the advantage of providing good adhesion and inflictinglittle impact stress on the target surface, as the sprayed material isliquid. The metal coatings may be cold-sprayed, in which case the metalwill be below its fusion temperature during spraying. Spraying metal atlow temperature has the advantage of inflicting less thermal stress onthe target surface, and allowing the newly coated target surface to behandled or moved soon after spraying. If the metal coating is thermallysprayed onto the target surface, it will be sprayed at or above thefusion temperature of the first, third, or fourth metal for the inner orintermediate coating, respectively.

The process may further comprise applying an underlayer to the substrateto improve adhesion of the inner coating to the substrate. Theunderlayer may be of any type or composition described above.

Regardless of whether thermal or cold spraying methods are used, themetal coating may be heated. Heating can occur prior to spraying, duringspraying, or after spraying to achieve different effects.

Heat to facilitate adhesion and mixing can also be provided byconduction from the target surface. The target surface may be heated forthe intended purpose of heating the first or third metal on contact toimprove adhesion, or the target surface may be heated as part of anotherprocess incidental to the coating process. One example of suchincidental heating is the residual heat from annealing. Many ferroussubstrates will be 400-800° F. (204-427° C.) immediately afterannealing, although a ferrous substrate may be at virtually anytemperature up to its melting point after annealing (for practicalconsiderations this will generally be no more than about 1800° F. (about980° C.)). In the case of iron, the substrate typically cools from 800°F. to 400° F. (427° C. to 204° C.); over such ranges the latent heat ofthe substrate will enhance the bonding of a zinc layer. The temperatureof the target surface may be, for example, about the fusing temperatureof the first, third or fourth metal. The temperature of the targetsurface may also be below the temperature of fusion of the first, thirdor fourth metal. For example, if the first metal were zinc, desirablesubstrate temperature ranges during spraying would be up to 850° F.(454° C.), 400-800° F. (204-427° C.), 400-500° F. (204-260° C.), orabout these ranges. In one example, a metallic layer of zinc is arcsprayed as a wire on to a substrate of 6″ ductile iron pipe while thesubstrate was at 250° F. (121° C.). In another example, a metallic layerof zinc was flame sprayed as a wire on to a substrate of 6″ ductile ironpipe while the substrate was at 850° F. (454° C.). In both examples theinner coating consisted of 200 g of zinc per m² of substrate surface.

2. Application of Intermediate Composite Coating

If the intermediate coating is a composite coating, it can be appliedusing any method known in the art, but advantageously will be applied byco-spraying the third metal and a second component (such as the fourthmetal or a metal oxide).

The third metal and second component may be sprayed simultaneously orsequentially. If sprayed sequentially, the earlier sprayed componentwill form a thin layer in which the later sprayed component is thenembedded. It is desirable that both components will be in substantialcontact with the inner coating. It is critical that the third metal besubstantially electrically connected to the substrate, via the innercoating or other means. In this context, “substantially electricallyconnected” means that a substantial portion of the third metal depositedcan convey a current to the substrate. If not electrically connected tothe substrate, the third metal cannot serve as a sacrificial anode. Suchelectrical connection can generally only be achieved through directcontact between the third metal 6 and the inner coating, although it ispossible that such a connection can be established across small gaps,particularly if such gaps contain an electrolyte.

If sprayed simultaneously, the third metal and second component may besprayed from the same spray head or spray heads, in which case they mustbe mixed prior to emission from the spray head. The feed material mayinvolve two separate feed streams, or it may comprise a single feedstream of mixed feed material (such as a wire or powder). Alternatively,the third metal and second component may be simultaneously sprayed fromdistinct spray heads; in this case, the components will not mix untilthey are aerosolized or until they impact the metallic layer. If sprayedfrom the same spray head, the third metal and second component will ofnecessity be at the same temperature when sprayed and the two componentswill of necessity be applied to the inner coating by the same spraymethod. The spraying technique may be any of those discussed above asuseful in the process, including cold spraying, warm spraying andthermal spraying. If sprayed from distinct spray heads, either componentmay be sprayed by any technique discussed above as useful in the processindependent of the other, including cold spraying, warm spraying andthermal spraying. The spray temperatures of each component will also beindependently selected.

If thermal spraying is used, the sprayed component will be heated to asuitable temperature. The temperature may be the fusion temperature ofthe sprayed component, or it may be above the fusion temperature. Ifboth components are sprayed at the same temperature (for example, whenemitted from the same spray head), the temperature may be at or abovethe fusion temperature of the component having the higher fusiontemperature. Thermal spraying techniques generally rely on the sprayedmaterial being in a liquid state, whereas in cold spraying and warmspraying the sprayed material may be in a solid state or a liquid state.Either approach may be used in the process.

In order to achieve optimum benefits and performance of the intermediatecomposite coating, it is desirable that the thickness of the coating,the size of the droplets, and the concentration of the droplets be in aratio designed such that the droplets in the coating are in contact withthe sacrificial metal coating of the inner coating. Droplet size of thesprayed component may vary considerably depending on the type ofmaterial being sprayed, the type of application utilized, and many othervariables. For example, the three most common methods of applyingthermally sprayed metals to pipe are electric arc spray, combustion gasspray utilizing wire, and combustion gas spray utilizing powder. Amongthe variables that affect the chosen droplet size of the third andsecond component are the form of the feed (wire, powder, liquid, etc.),size of the feed particles, feed rate, air delivery pressure, combustiongas pressures (for combustion gas spray), voltage utilized (for electricarc spray), type of spray gun utilized, spray tip configuration, spraytip size, distance from the spray gun tip to the part being sprayed, andambient conditions.

It is necessary that the thickness of the intermediate composite coatingbe properly designed based on metal particle size and configuration andthe concentration of the metal particles dispersed in the intermediatecomposite coating. One object is to ensure that the third metal will besubstantially electrically connected to the substrate. This can beachieved, for example, by ensuring that the thickness of the layer doesnot exceed the average diameter of the individual metal particles. Asanother example, the same goal can be achieved by ensuring that thedesirable matrix configuration is such that the size and concentrationof the metal droplets are sufficient to result in an electricallycontinuous conductive path from one side of the intermediate compositecoating to the other.

Prior to the application of the metal coating, the target surface may beabrasive blast cleaned. Abrasive blast cleaning can be used to removecontaminants while allowing protective oxides to remain on the targetsurface. Abrasive blast cleaning also can serve to increase adhesionbetween the target surface and the metal coating by abrading the targetsurface. Blast cleaning is more suitable for the metallic substrate thanfor example the first coating, as the metallic substrate will in mostcases be thicker and more durable than the first coating.

3. Application of Outer Coating

Some embodiments of the process comprise applying the outer coating toat least one of the inner coating or intermediate coating. The outerlayer may be applied by any means known in the art, but it isparticularly advantageous to apply the outer layer by the advanced spraymethods disclosed herein.

Applying the outer layer provides a barrier layer without need forfurther assembly or application when a metal structure is installed.This has significant advantages over previous methods, which oftenrequire that coverings or sheaths be placed over surfaces or pipes priorto installation. Such methods are vulnerable to errors in deployment andaccidents that can result in incorrect use of the barrier or damage tothe outer coating. Some advantages of applying the outer coating viaadvanced spray methods are that it can be performed duringmanufacturing, it can be performed quickly, it can be performed at lowcost, and it can provide thin and consistent layering.

Thermal spraying is a suitable method for applying the outer coating.Because of the high temperatures of fusion of many metal oxides, plasmaspraying is a particularly suitable method. The spraying of the outercoating will be subject to the same parameters and requirements asdiscussed above for the spraying of the inner coating and theintermediate coating.

B. Examples

An embodiment of the coating was applied to a ductile iron pipe forfield testing. The pipe was heated to about 500° F. prior to applicationof the inner coating. An inner coating of over 99% metallic zinc wasapplied to the ductile iron pipe by thermal spraying (arc spraying)using a Miller Thermal PF40 arc spray unit. An intermediate coatingcomprising over 99% metallic aluminum was applied to the inner coatingby the same method. An outer coating of Al₂O₃ was applied to theintermediate coating by thermal spraying (flame spraying) using aSulzer-Metco 5P-II thermospray flame spray unit. An asphalt topcoat wasthen applied to the outer coating.

The coated metal pipe was buried in an aggressive subtropical wetlandsoil for one year. An uncoated ductile iron pipe was buried at the samesite for the same period as a negative control. After one year, bothpipes were excavated an examined. The sample with the protective coatingshowed no corrosion, whereas the uncoated ductile iron pipe was highlycorroded.

The same coated metal pipe has been tested in the laboratory byimmersion in 5% NaCl solution over long periods. Coated pipe immersed in5% NaCl for nine months at room temperature significantly outperformedductile iron having only an asphalt coating, as did coated pipe exposedto a constant 1 hour wet/dry cycle in 5% NaCl for seven months at roomtemperature.

C. Conclusions

The foregoing description illustrates and describes the processes,machines, manufactures, compositions of matter, and other teachings ofthe present disclosure. Additionally, the disclosure shows and describesonly certain embodiments of the processes, machines, manufactures,compositions of matter, and other teachings disclosed, but, as mentionedabove, it is to be understood that the teachings of the presentdisclosure are capable of use in various other combinations,modifications, and environments and is capable of changes ormodifications within the scope of the teachings as expressed herein,commensurate with the skill and/or knowledge of a person having ordinaryskill in the relevant art. The embodiments described hereinabove arefurther intended to explain certain best modes known of practicing theprocesses, machines, manufactures, compositions of matter, and otherteachings of the present disclosure and to enable others skilled in theart to utilize the teachings of the present disclosure in such, orother, embodiments and with the various modifications required by theparticular applications or uses. Accordingly, the processes, machines,manufactures, compositions of matter, and other teachings of the presentdisclosure are not intended to limit the exact embodiments and examplesdisclosed herein.

We claim:
 1. A process for coating a ductile iron surface, comprising:(a) thermally spraying a first spray onto the ductile iron surface, thefirst spray comprising at least 95% w/w metallic zinc, to form an innergalvanizing coating that is anodic relative to the ductile iron surfaceand in direct contact with the ductile iron surface; and (b) thermallyspraying a second spray onto the inner coating, the second spraycomprising at least 90% w/w Al₃O₂, to form an outer dielectric coatingin direct contact with the inner coating; wherein the sum of thethicknesses of the inner and outer coatings is at most 10 mils (254 μm).2. A process for coating a metal surface, comprising: (a) thermallyspraying a first spray to form a galvanizing inner coating substantiallyelectrically connected to the metal surface, wherein (i) the first spraycomprises a first metal that is anodic to the metal surface, and (ii)the inner coating is anodic to the metal surface; and (b) thermallyspraying a second spray to form a dielectric outer coating on the sideof the inner coating opposite the metal surface, wherein the secondspray comprises a dielectric oxide of a second metal.
 3. The process ofclaim 2, wherein the first spray is thermally sprayed directly onto themetal surface.
 4. The process of claim 2, wherein the second spray isthermally sprayed directly onto the inner coating.
 5. The process ofclaim 2, wherein the first metal is selected from the group consistingof: zinc, aluminum, magnesium, indium, gallium, and tellurium.
 6. Theprocess of claim 2, wherein the first spray comprises at least 95% w/wof the first metal, which is selected from the group consisting of:zinc, aluminum, magnesium, indium, gallium, tellurium, or a combinationof any of the foregoing.
 7. The process of claim 2, wherein the firstspray comprises at least 85% w/w of the first metal, which is zinc. 8.The process of claim 2, wherein the first spray comprises at least 95%w/w of the first metal, which is zinc.
 9. The process of claim 2,wherein the first metal is zinc and the oxide of the second metal is anoxide of aluminum.
 10. The process of claim 2, wherein the inner coatingis spayed to a thickness of about 2-3 mils (50.8-76.2 μm).
 11. Theprocess of claim 2, wherein the outer coating is sprayed to a thicknessof about 2-6 mils (50.8-152.4 μm).
 12. The process of claim 2, whereinthe sum of the thicknesses of the inner coating and the outer coatingdoes not exceed about 10 mils (254 μm).
 13. The process of claim 2,wherein the oxide of the second metal is selected from the groupconsisting of: an oxide of aluminum, an oxide of magnesium, and an oxideof titanium.
 14. The process of claim 2, wherein the second spraycomprises at least 95% w/w of the oxide of the second metal.
 15. Theprocess of claim 2, wherein the second spray comprises at least 95% w/wof the oxide of a second metal, which is selected from the groupconsisting of: zinc, aluminum, magnesium, titanium, or a combination ofany of the foregoing.
 16. The process of claim 2, wherein the secondspray comprises at least 95% w/w Al₂O₃.
 17. The process of claim 2,comprising applying an organic topcoat outside of the outer coating. 18.The process of claim 2, comprising applying an organic topcoat outsideof the outer coating, and the topcoat is selected from the groupconsisting of: asphalt, paint, and wax.
 19. The process of claim 2,comprising: thermally spraying a third spray prior to forming the outercoating, to form an intermediate galvanizing coating between the innercoating and the outer coating, wherein the second spray comprises athird metal that is anodic to the metal surface.
 20. The process ofclaim 19, wherein the intermediate coating is anodic to the metalsurface.
 21. The process of claim 19, wherein the intermediate coatingis anodic to the metal surface and the inner coating.
 22. The process ofclaim 19, wherein: (a) the first spray comprises at least 80% w/w of thefirst metal which is zinc; (b) the third spray comprises at least 90%w/w of the third metal which is selected from aluminum, titanium, or amixture of both; and (c) the second spray comprises at least 90% w/wAl₂O₃.
 23. The process of claim 19, wherein the third metal is aluminum.24. A coated metal surface that is the product of the process of claim2.
 25. A fluid conduit comprising the coated metal surface that is theproduct of the process of claim 2.